Recently, a virtualization function called Thin Provisioning has been drawing attention as one of functions installed in a storage apparatus. The thin provisioning function is a function providing a virtual logical volume (hereinafter referred to as a virtual volume) to a host computer and dynamically allocating a storage area to the virtual volume if a data write request is made from the host computer to the virtual volume. Such a thin provisioning function has advantages that make it possible to provide a virtual volume of a larger capacity than the capacity of a storage area, which can be actually provided, to the host computer, reduce a physical storage capacity in the storage apparatus which must be prepared in advance, and construct a computer system at a low cost.
Furthermore, when the above-described thin provisioning function is utilized and the host computer deletes data, if only management information of the relevant data is deleted without deleting the actual data, there are problems, that is, the storage apparatus cannot recognize a storage area where unnecessary data is stored, and the storage area allocated to the virtual volume once cannot be released. Therefore, Patent Literature 1 discloses a technology of allowing the storage apparatus to recognize a storage area, where the unnecessary data is stored, and releasing the relevant storage area even if the data management information is deleted without deleting the actual data.